1. Field of the Invention
The present invention relates to a method for pulling a single crystal and, more particularly, to a method for pulling a single crystal wherein a single crystal of silicon or the like is pulled from a melt of a material for forming a single crystal by a pulling method such as the Czochralski method (hereinafter, referred to as the CZ method).
2. Description of the Related Art
At present, the majority of silicon single crystals used for manufacturing a substrate for forming a circuit component of a LSI (large scale integrated circuit) and the like have been pulled by the CZ method. FIG. 1 is a diagrammatic sectional view of an apparatus for pulling a single crystal used for the CZ method, and in the figure, reference numeral 21 represents a crucible.
The crucible 21 comprises a bottomed cylindrical quartz crucible 21a and a bottomed cylindrical graphite crucible 21b fitted on the outer side of the quartz crucible 21a. The crucible 21 is supported with a support shaft 28 which rotates in the direction shown by the arrow A in the figure at a prescribed speed. A heater 22 of a resistance heating type and a heat insulating material 27 arranged around the heater 22 are concentrically arranged around the crucible 21. The crucible 21 is charged with a melt 23 of a material for forming a crystal which is melted by the heater 22. On the central axis of the crucible 21, a pulling axis 24 made of a pulling rod or wire is suspended, and at the front thereof, a seed crystal 25 is held by a holder 24a. These parts are arranged within a water cooled type chamber 29 wherein pressure can be controlled.
A method for pulling a single crystal 26 using the above-mentioned apparatus for pulling a single crystal is described below by reference to FIGS. 1 and 2. FIGS. 2(a)-(d) are partial magnified front views diagrammatically showing the seed crystal and the vicinity thereof in each step in pulling a single crystal.
Although it is not shown in FIG. 2, an inert gas is induced into the chamber 29 so as to make an inert gas atmosphere under reduced pressure, and then, the material for forming a crystal is melted by the heater 22. The situation is maintained for a period of time so as to sufficiently release gas contained in the melt 23.
While the pulling axis 24 is rotated on the same axis in the reverse direction of the support shaft 28 at a prescribed speed, the seed crystal 25 held by the holder 24a is caused to descend and is brought into contact with the melt 23 so as to make the seed crystal 25 partially melt into the melt 23. Then, the pulling of the single crystal 26 is started (hereinafter, referred to as the seeding step) (FIG. 2(a)).
In making a single crystal grow at the front of the seed crystal 25, the pulling axis 24 is pulled away from the melt at a higher speed than the below-described pulling speed in forming a main body 26c. The crystal is narrowed down to have a prescribed diameter, leading to the formation of a neck 26a (hereinafter, referred to as the necking step) (FIG. 2(b)).
By slowing down the pulling speed of the pulling axis 24 (hereinafter, simply referred to as the pulling speed), the neck 26a is made to grow to have a prescribed diameter, leading to the formation of a shoulder 26b (FIG. 2(c)).
By pulling the pulling axis 24 at a fixed rate, the main body 26c having a uniform diameter and a prescribed length is formed (FIG. 2(d)).
Although it is not shown in FIG. 2, in order to prevent induction of high density dislocation to the single crystal 26 by a steep temperature gradient in the last stage, the diameter thereof is gradually decreased and the temperature of the whole single crystal 26 is gradually lowered, leading to the formation of an end-cone. Then, the single crystal 26 is separated from the melt 23 and is cooled, which is at the end of the pulling of the single crystal 26.
One of the most important steps in the pulling of the single crystal 26 is the above-mentioned necking step (FIG. 2(b)). The object of the necking step is described below. Before the seeding step (FIG. 2(a)), the seed crystal 25 is caused to once stop immediately above the melt 23, is preheated by the melt 23, and is then brought into contact with the melt 23. There is usually a difference of 100.degree. C. or more between the temperature of the seed crystal 25 after preheating (about 1300.degree. C. and less) and the melting point of the seed crystal 25 (about 1410.degree. C.). Therefore, when the seed crystal 25 is brought into contact with the melt 23, the seed crystal 25 has a steep temperature gradient, leading to the induction of the dislocation caused by a thermal stress to the lower portion 25b of the seed crystal 25. It is necessary to make the single crystal 26 grow after excluding the dislocation which inhibits single crystal growth. Since the dislocation generally grows in the vertical direction to the growth interface of the single crystal 26, the shape of the growth interface (the front plane of the neck 26a) is made to be downward convex in the necking step, so as to exclude the dislocation outward.
In the necking step, the faster the pulling speed, the smaller the diameter of the neck 26a becomes and the more downward convex the shape of the growth interface becomes. As a result, the dislocation can be inhibited from propagating and can be efficiently excluded.
In the above conventional method for pulling a single crystal, the seed crystal 25 having a diameter of about 12 mm has been generally used in order to pull the single crystal 26 having a diameter of about 6 inches and a weight of 80 kg or so. The larger the diameter of the neck 26a, the more safely the single crystal 26 is held, while the smaller the diameter of the neck 26a, the more efficiently the dislocation is excluded. In order to meet both of the requirements, the neck 26a having a diameter of about 3 mm has been selected. Recently, however, in order to manufacture a more highly integrated semiconductor device at a lower cost and more efficiently, the wafer has been required to have a larger diameter. Now, for example, the production of the single crystal 26 having a diameter of about 12 inches (300 mm) and a weight of 300 kg or so is desired. In this case, the neck 26a having a conventional diameter (usually 3 mm or so) cannot withstand the weight of the pulled single crystal 26 and breaks, resulting in the falling of the single crystal 26.
In growing the above heavy single crystal 26, the diameter of the neck 26a needs to be about 6 mm or more, in order to prevent the occurrence of troubles, such as a fall of the single crystal 26 and to pull the single crystal 26 safely, which diameter is calculated from the silicon strength (about 16 kgf/mm.sup.2). However, when the diameter of the neck 26a is 6 mm or more, the dislocation induced in dipping the seed crystal 25 into the melt 23 cannot be sufficiently excluded using known methods.